The present invention relates to silicon carbide (SiC) fibers used as reinforcement in the fabrication of composite materials, and more particularly thermostructural composite materials made of fiber reinforcement densified by a matrix.
Thermostructural composite materials are characterized by their mechanical properties that make them suitable for constituting structural parts, and by their capacity for conserving these mechanical properties at high temperatures. Nevertheless, ceramic fibers, such as silicon carbide (SiC) fibers, are sensitive to oxidation when they are exposed at high temperatures (e.g. in the range 300° C. to 1500° C.) to an oxidizing medium, in particular in the presence of air, or water vapor, and more generally in the presence of any gaseous or liquid phase containing oxygen or an oxygen compound.
The oxidation of SiC fibers has a direct impact on their lifetime, and consequently on the lifetime of the composite material for which they constitute the reinforcement.
Document WO 2010/076475 describes a method of treating ceramic fibers, which method comprises first heat treatment in a reactive gas performed using at least one first reactive gas of halogen type that acts by chemically transforming the surface of the fiber to form a surface layer constituted for the most part of carbon, and second heat treatment in a reactive gas performed with at least one second reactive gas that eliminates the surface layer formed on the surface during the chemical transformation. Using those two heat treatments with different and appropriate reactive gases makes it possible to totally eliminate the surface layer of the material of the fibers, which layer contains the defects that have the greatest influence on limiting the mechanical properties and the lifetime of the fibers. Nevertheless, that treatment method does not improve the ability of the fibers to withstand oxidation.
Heat treatment of ceramic fibers with a halogen gas is also used in Document WO 2005/092610 as an intermediate step in a method of making a boron aluminum nitrogen (BAN) type coating on ceramic fibers, the coating being constituted for example by a mixture of BN and of Al(O)N, for the purpose of improving the ability of ceramic composites to withstand oxidation.
Although ceramic fibers provided with such a coating present an improvement in their ability to withstand oxidation, that ability nevertheless remains insufficient, in particular in terms of increasing the lifetime of the fibers.
There thus exists a need to protect SiC fibers individually against oxidation.